1. Field of the Invention
The present invention relates to a method of manufacturing a correction lens for forming a phosphor screen on a faceplate of a color cathode ray tube.
2. Description of the Related Art
As shown in FIG. 1, a color cathode ray tube generally has an envelope constituted by a panel 1 and a funnel 2. A phosphor screen 4 is formed on the inner surface of a faceplate of the panel 1 to oppose a shadow mask 3 mounted inside the panel a large number of apertures for passing electron beams. The phosphor screen 4 is formed of stripe- or dot-like 3-color phosphor layers for emitting blue, red, and green light rays. In order to improve contrast of an image formed on the phosphor screen 4, non-light-emitting layers mainly consisting of carbon may be formed between the 3-color phosphor layers. A screen of this type is known as a so-called black stripe type or black matrix type screen.
Three electron beams 6B, 6G, and 6R emitted from an electron gun assembly 5 are horizontally and vertically deflected by a magnetic field generated by a deflection yoke 7 mounted outside the funnel 2, and the phosphor screen 4 is scanned by the deflected electron beams, thereby displaying an image on the screen 4.
In order to display an image having high color purity on the phosphor screen 4, as shown in FIG. 2, the electron beams 6B, 6G, and 6R passing through a given aperture 8 formed in the shadow mask 3 must be correctly incident on corresponding phosphor layers 9B, 9G, and 9R, respectively. As the electron beams 6B, 6G, and 6R are deflected, i.e., deflection angles of the beams are changed, centers of deflection of apparent electron beams are changed along the tube axis. Therefore, the 3-color phosphor layers 9B, 9G, and 9R must be formed at specific aligning pitches for each aperture 8 of the shadow mask 3. That is, in order to allow the electron beams 6B, 6G, and 6R to be correctly incident on the corresponding phosphor layers 9B, 9G, and 9R, respectively, formation positions of the phosphor layers 9B, 9G, and 9R with respect to the apertures 8 of the shadow mask 3 must be continuously changed on the inner surface of the panel 1.
FIG. 3 shows an orbit of the center beam 6G of the three electron beams aligned in a line emitted from an in-line type electron gun assembly. As shown in FIG. 3, assuming that the magnetic field intensity of the deflection coil 7 is uniform, the electron beam 6G propagates along a substantially circular orbit in a deflection yoke magnetic field 11, propagates straight from the magnetic field 11, and is incident on the phosphor layer 9G through the aperture 8 of the shadow mask 3. Therefore, an apparent emission position of the electron beam 6G, i.e., a position of a deflection center (F) at which an extended line of a straight orbit crosses the tube axis (X axis) is changed in accordance with a deflection angle .gamma.. That is, the cathode ray tube apparatus has a .gamma.-.DELTA.p characteristic in which the deflection center (F) moves toward the phosphor screen 4 by .DELTA.p when the deflection angle is .gamma. with respect to a deflection center O obtained when the deflection angle is zero, i.e., in a non-deflection state.
In a conventional method, a phosphor slurry mainly consisting of a phosphor and a light-sensitive resin is coated and dried on the inner surface of a panel, the obtained film is exposed through a shadow mask to bake patterns corresponding to all apertures formed in the shadow mask, and the resultant material is developed to remove a non-exposed portion, thereby forming a phosphor layer of an arbitrary color. The above steps are repeatedly performed for phosphor layers of three colors to form a phosphor screen. Especially in a black stripe or black matrix type phosphor screen, prior to formation of 3-color phosphor layers, a light-sensitive resin is coated on the inner surface of the panel to form a pattern of the light-sensitive resin corresponding to the apertures of the shadow mask in a prospective 3-color phosphor layer formation region by a method similar to formation of the phosphor layers as described above. Thereafter, a non-light-emitting paint is coated, and the film of this non-light-emitting paint is removed together with the light-sensitive resin pattern, thereby forming non-light-emitting layers with gaps in the prospective 3-color phosphor layer formation position. Thereafter, the 3-color phosphor layers are formed between the non-light-emitting layers by the above phosphor layer formation step to form a phosphor screen.
In an exposure step for any of the phosphor layers and the non-light-emitting layers, light rays for exposing the film formed on the panel inner surface propagate straight. Therefore, as shown in FIG. 4, a correction lens 15 is arranged between the panel 1 mounting that encloses the shadow mask 3 and an exposure light source 13 to approximate an orbit of light rays 14 emitted from the light source 13 to an orbit of electron beams emitted from the electron gun assembly.
A spherical lens is conventionally used as the correction lens 15. As the structure of the color cathode ray tube is complicated, however, a simple lens cannot create the .gamma.-.DELTA.p characteristic. Therefore, an aspherical lens having a complicated surface shape is currently used.
If the surface shape of the aspherical lens is represented by an orthogonal coordinate system (X,Y,Z) having the center of the bottom surface of the lens as its origin, a surface height x at a given point is represented by: EQU x=f(y,z) (1)
In a polar coordinate system (r,.theta.), x is represented by: ##EQU1## In general, equation (1) is represented by the following polynomial: ##EQU2##
In design of the correction lens performed by using the above equations, a change in the light rays emitted from the exposure light source with respect to a change amount of the coefficient a.sub.ij is tracked throughout the entire surface of the phosphor screen. As a result, the correction lens is designed such that errors between incident positions of the electron beams on the entire surface of the phosphor screen and a phosphor layer on which the electron beams are to be landed are set to be a predetermined value or less, normally, 10 .mu.m or less. When the correction lens is designed by this method, errors at a limited small number of points on the correction lens can be comparatively easily reduced. However, even if the coefficient a.sub.ij is set to reduce an error at a given point on the correction lens surface, the coefficient a.sub.ij generally functions to increase errors at most of other points. Therefore, it is very difficult to design the correction lens such that errors at all the points on the phosphor screen are set to be a desired value or less. Even if a high-performance high-speed computer is used, not only design is time-consuming, but also changing or setting of the coefficient a.sub.ij requires determination based on rich experiences in many cases.
In a color cathode ray tube having a complicated deflection magnetic field such as a 110.degree.-deflection, a color cathode ray tube having a large deflection angle, or a large-size color cathode ray tube, it takes a long time to design the correction lens, and it is difficult to manufacture the correction lens having a desired .gamma.-.DELTA.p characteristic.
Published Examined Japanese Patent Application No. 47-40983 or 49-22770 discloses another correction lens designing method. In this method, as shown in FIGS. 5A and 5B, an effective surface of a correction lens 15 is divided into a plurality of regions, and inclination of the surface is determined for each region. In this method, since an orbit of light rays can be precisely approximated to an orbit of electron beams in units of divided regions, a correction lens which comparatively satisfies the .gamma.-.DELTA.p characteristic can be made.
In this correction lens 15, however, steps 17 are formed in boundary portions between the divided regions. Therefore, exposure variation is easily produced by non-uniformity in light amount caused by the steps 17 on a black stripe or black matrix type phosphor screen in which non-light-emitting layers are formed between the 3-color phosphor layers. In order to solve this problem, the correction lens 15 may be tilted or the steps may be shielded from light during exposure. In either method, however, the exposure variation on the phosphor screen cannot be satisfactorily eliminated.
The present inventor, therefore, proposed a correction lens in Japanese Patent Application No. 63-247192. In this correction lens, an effective surface of the lens is divided into a plurality of regions similar to the correction lens having a plurality of regions, the .gamma.-.DELTA.p characteristic is set to minimize a landing error for each region, and no step is formed in boundary portions between the regions.
When such a correction lens is used, a phosphor screen which satisfies landing characteristics and is free from exposure variation can be formed without tilting a light source. Since, however, this correction lens requires surface equations representing surface shapes in a number corresponding to the number of divided regions, a long time and a lot of labors are required in the manufacture of the correction lens.
As described above, in a phosphor screen of a color cathode ray tube, when a pattern corresponding to apertures of a shadow mask is to be baked on a film formed PG,9 on the inner surface of a panel and consisting of a phosphor slurry or a light-sensitive resin, a correction lens for approximating an orbit of light rays radiated from an exposure light source to that of electron beams deflected by a magnetic field formed by a deflection yoke is used. Since, however, the surface shape of this correction lens is complicated, it is difficult to design a correction lens so that good landing is obtained throughout the entire phosphor screen. Therefore, a desired correction lens cannot be manufactured for a phosphor screen of especially a color cathode ray tube having a complicated deflection magnetic field such as a color cathode ray tube having a wide deflection angle or a large-size cathode ray tube.
As a correction lens for solving the above problem, a correction lens in which its effective surface is divided into a plurality of regions so that each region satisfies a .gamma.-.DELTA.p characteristic is disclosed in each Published Examined Japanese Patent Application Nos. 47-40983 and 9-22770 and Japanese Patent Application No. 63-247192.
Of these correction lenses having divided regions, since each of the correction lenses disclosed in Published Examined Japanese Patent Application Nos. 47-40983 and 49-22770 has steps in boundary portions between the divided regions, exposure variation is easily produced due to non-uniformity in light amount caused by the steps. Since, however, the correction lens disclosed in Japanese Patent Application No. 63-247192 proposed by the present inventor has no step in boundary portions between the divided regions, a phosphor screen free from exposure variation can be manufactured. However, this correction lens requires surface equations representing surface shapes in a number corresponding to the number of the divided regions. Therefore, a long time period and a lot of labors are required in the manufacture of this correction lens.